add the capability for index-pack to read from a stream

This patch only adds the streaming capability to index-pack.  Although
the code is different it has the exact same functionality as before to
make sure nothing broke.

This is in preparation for receiving packs over the net, parse them on
the fly, fix them up if they are "thin" packs, and keep the resulting
pack instead of exploding it into loose objects.  But such functionality
should come separately.

One immediate advantage of this patch is that index-pack can now deal
with packs up to 4GB in size even on 32-bit architectures since the pack
is not entirely mmap()'d all at once anymore.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>